OPTICS GLOSSARY

A BRIEF EXPLANATION OF CERTAIN OPTICAL TERMS OPTICAL MEASURING

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PRE1305Optics glossary.indd 1 01.12.14 13:40 PRE1305Optics glossary.indd 2 01.12.14 13:40 INTRODUCTIONINTRODUCTION

With seven decades of experience and more than 5,000 specialized products, Mitutoyo is both pioneer and pace setter in the field of precision length measurement technology around the globe.

In addition to coordinate and form measuring machines, sensor systems, hardness testing equipment, linear scales and hand-held measuring devices, the better known products also include optical measuring equipment, such as measuring projectors, measuring microscopes and vision measuring systems. Few people are, however, aware that Mitutoyo develops and manufactures the full-range of optics for this equipment itself – so it is not only competent in the field of length measurement technology but also equally expert at optical systems.

Unfortunately, a user is also expected to demonstrate the same level of competence in both fields – metrology and optics – if measuring equipment with optical sensors is to be used properly and – even more important – the ensuing results are to be interpreted correctly.

This glossary is designed for classic measurement engineers, i.e., for users with in-depth knowledge of metrology but who only have some basic knowledge – possibly dating back to their school days - of optical issues.

They will find brief explanations of individual optical terms in this glossary to help them bring their level of knowledge back up to par. As such, the glossary aims to enable them to identify and evaluate the impacts of optical phenomena on the measuring results generated by optical length measurement equipment.

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PRE1305Optics glossary.indd 3 01.12.14 13:40 CONTENTSCONTENTS

Page Page

Aberration 7 Transmittent light 34 Balance length 8 Built-in microscope 35 Achromat / Achromatism 9 Entrance pupil 36 Aperture diaphragm 10 Eclipse / Shadowing / Vignetting 36 Apochromat 10 Finitely corrected optical system 37 Working distance 11 Color temperature 38 Astigmatism 12 Color magnification error 39 Resolution 13 Field diaphragm 39 Resolving power 13 Flickering 40 Eye position 15 Fluorescence ring light 40 Exit pupil 16 Fluorescence analysis 41 Axial chromatic aberration 18 Fluorescence microscope 41 Field curvature 19 Focus, focus point / Focal point 42 Image position / Intermediate image position 20 Focal width / Focal length 42 Vision measurement 21 Galileian type / Parallel type (stereo micro- 43 Refraction 22 scope) Law of refraction 23 Ghost image 43 Refraction index 24 Total magnification 44 CCD 25 Greenough type (stereo microscope) 44 Chromatic aberration 26 Green GIF filter 45 CMOS 27 Half mirror / Semi-permeable mirror 46 C-Mount 28 Principal point / Principal plane 47 Deviating mirror / Deviating prism 29 Principal ray 48 Dichroic mirror 29 Bright field analysis 49 Differential interference contrast 30 Infrared microscope 50 Dual image 32 Interference 51 Dual refraction 32 Inverted microscope 51 Dark field analysis 33 Coaxial incident light 52

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PRE1305Optics glossary.indd 4 01.12.14 13:40 CONTENTS

Page Page Koehler illumination 52 Reflection 77 Collimator (lens) 53 Ring light 78 Coma 53 Depth of field 79 Condenser (lens) 54 Field number 80 Confocal microscope 55 Visible area 80 Laser 56 Seidel aberrations 81 LB filter 57 True to side image 81 LED lighting 58 Inverted image 81 Light 59 Visible light 82 Light microscope 60 Spectral properties 82 Wavelength of light 61 Spherical aberration 83 Measuring microscope 62 Stereo microscope 84 Metallurgical microscope 63 Telecentric lens 85 Near-infrared radiation 64 Telecentric illumination 86 Near-ultraviolet radiation 64 Total reflection 86 ND filter 65 Transmission factor / Aperture factor 88 Numerical aperture 66 Tube lens 89 Nomarski prism 67 Ultraviolet radiation 90 Subject-image distance 68 Ultraviolet microscope 91 Objective lens 69 Infinitely corrected optical system 92 Eyepiece 70 Magnification uncertainty 92 Optically active materials 71 Distortion 93 Optical density 71 Virtual image 94 Plane correction 72 White balance 94 Polarization 73 Wollaston prism 95 Polarized analysis 74 Centering 95 Prism 75 Point diameter / Point size 76

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PRE1305Optics glossary.indd 6 01.12.14 13:40 Aberration

This is the generic term used to describe imaging errors. Imaging errors are caused by the widest range of different optical effects and result in deterioration in the quality of the image of the work piece, making it appear, for example, unfocused, warped or distorted. These aberrations must be eliminated as far as possible in microscopy since the image must correspond to the work piece if it is to be used to evaluate the work piece. A distinction is made between geometric aberrations, which are caused by the geometric – i.e. spherical - shape of the lens - and chromatic aberrations (chromatic = relating to color), which are caused by different strengths of refraction of the individual wavelengths of light – or colors. The five primary geometric aberrations are also known as “Seidel aberrations”, named after the physicist Philip Ludwig von Seidel, who examined the various types of aberration for the first time back in the second half of the nineteenth century.

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PRE1305Optics glossary.indd 7 01.12.14 13:40 Balance length / Parfocal length

A work piece frequently has to be examined at several different levels of magnification in order to perform various microscopy tasks. An objective revolver is generally used for this purpose, which enables objective lenses with different levels of magnification to be easily moved into the microscope’s light path. In order to significantly raise the benefit for the user, it would be helpful if an area that has already been sharply focused using one objective lens could remain sharp when a different level of magnification is set. This is achieved by balancing the length of the objective lenses in revolvers. Balanced – or par- focal – objective lenses are always located at the same distance from the flange of the thread used to attach them to the revolver to the work piece. If the working distance between the objective lens and the work piece is enlarged, the length of the objective lens itself declines accordingly. The aggregate of both lengths always remains the same and is indicated as the balance or parfocal length in technical documentation.

W.D: Working Distance P.D. P.D. P.D: Parfocal Distance σ: Half objective beam width  W.D W.D

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PRE1305Optics glossary.indd 8 01.12.14 13:40 Achromat / Achromatism

If several lenses are combined into a small lens system, the axial chromatic aberration caused by one lens can be compensated by the aberration of another lens. This ensures that the rays of all wavelengths or colors meet up in a single focal point. In the case of more complex lens systems and objective lenses it is, however, not possible to fully compensate the errors caused by all lenses. Only the red and blue rays of light are analyzed in this case, since they form the beginning, respectively, the end of the visible wavelength spectrum. So if their focal points are corrected to ensure that they are in the same position at the end, the focal points of the other colors between them can be assumed to be negligibly close to this position. As such the axial chromatic aberration is minimized. This procedure for minimizing axial chromatic aberration is known as achromatism, a lens system (such as an objective lens) that has been corrected accordingly, is called an achromat.

Schematic diagram of an achromat

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PRE1305Optics glossary.indd 9 01.12.14 13:40 Aperture diaphragm

An aperture diaphragm is a diaphragm with an adjustable opening that influences the light intensity and resolving power thanks to its precisely defined position in the microscope’s light path. In the case of measuring microscopes, the aperture can be used to ensure maximum accu- racy when measuring pins or waves. If the opening diameter of the diaphragm is optimally adjusted, the marked diffraction effects that otherwise occur on the surfaces can be largely eliminated.

Aperture diaphragm diameter (mm)

DIAPHRAGM 10 15 25 5 20 1 25

20

15

10

5

0510 15 20 25 30 35 40 45 50 Work piece diameter d (mm)

Apochromat

The achromatic correction of axial chromatic aberrations is fully sufficient for many analysis purposes. Since achromats are, moreover, comparatively simple and therefore cheap to manufacture, they are frequently used in microscopes. However, achromatic correction, i.e., the correction of red and blue light in a common focal point, is sometimes not sufficient for certain applications. In such cases, a further light – yellow – is corrected in the common focal point together with the light paths of the red and blue lights as the wavelength of yellow light is relatively equidistant to the wavelengths of red and blue light.

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PRE1305Optics glossary.indd 10 01.12.14 13:40 This apochromatic correction produces an even better quality than achromatic correction and eliminates the axial chromatic aberration virtually entirely. Apochromats are, however, much harder to manufacture and therefore, of course, more expensive.

Working distance

The term working distance is used to describe the distance between the lowest point of the objective lens and the imaged area of the work piece. A larger working distance has proven its worth, especially when performing analyses under non-laboratory conditions. On the one hand, it allows the work piece or specimen to be positioned and removed quickly and simply without risking damage to the work piece or objective lens. The risk of damage when sharpening or adjusting the focus on the image is also minimized, given that the objective lens could easily touch the work piece by accident at a small working distance. Specimens generally also do not need to be prepared in advance if the working distance is large, surfaces do not have to be as flat as possible, since different heights or steps on the work piece are irrelevant as long as they fit within the working distance. On the other hand, a large working distance also enlarges the depth of field. For purely ana- lytical purposes, this may be advantageous, but it represents a major drawback when using a microscope to measure lengths. On the one hand, it increases the uncertainty of height meas- urements, while on the other hand distortions in perspective could result in incorrect results, which is not the case with a small depth of field as the image immediately goes out of focus. In addition, the optical resolving power is reduced at a large working distance, resulting in much less focused and opaque images, especially at large magnifications, and preventing the clear detection of small details. As such, the aim cannot be to achieve as large a working distance as possible, but rather to identify the best possible compromise between working distance and resolving power.

P.D. P.D. W.D: Working Distance P.D: Parfocal Distance  W.D W.D σ: Half objective beam width

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PRE1305Optics glossary.indd 11 01.12.14 13:40 Astigmatism

Astigmatism is an imaging error and one of the 5 Seidel aberrations caused by the geo- metric shape of the lens.In the case of astigmatism, the curvature of the lens is not identical at its edges. For example, the upper section of the lens could already be strongly curved on the perpendicular plane, whereas the curvature on the horizontal plane is still only minor. Accordingly, the refraction of the light rays is more pronounced in this area in the perpen- dicular than in the horizontal. As such, light emanating from a point and touching the edge of the lens is not evenly refracted and therefore does not merge into a common focal point.

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PRE1305Optics glossary.indd 12 01.12.14 13:40 Resolution

Resolution is the term used to define the minimum requisite optical resolving power needed to distinguish between two objects, such as points or lines on the image.

Resolving power

a. See Optical resolving power

The optical resolving power describes the smallest possible distance between two points or lines at which an optical system (e.g. microscope) can still just identify them as separate objects on the image. The term is unfortunately somewhat misleading, since our understanding of the language would instinctively mean that a microscope should have as high a resolving power as possible. But in terms of distance, a system logically offers much better resolution at the smallest pos- sible distance between the objects. In other words, the resolution of an optical system improves as soon as the resolving power becomes smaller. Optical resolving power is limited by interferences that occur at small distances as a result of diffraction. These interferences prevent a point from being clearly shown as a point. Instead, further rings of light form around the mapped point. As soon as these diffraction rings surrounding two adjacent points overlap, the points can no longer be distinguished as individual points. The strength of diffraction, in turn, depends on the optical density of the medium between the subject and the optical system and on the wavelength of the light source used. The objec- tive lens must be capable of capturing the interference rings, which is logically limited by the maximum angle of incidence to the objective lens. The calculation of the optical resolving power is therefore logically dependent on these three factors:

λ 1 λ = Wavelength of the light source used γ= * 2 n* N.A. n = Refraction index of the medium between the subject and the optical system N.A. = Numerical aperture of the optical system

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PRE1305Optics glossary.indd 13 01.12.14 13:40 The medium between the subject and the microscope’s objective lens is generally air, and since air has a refraction index of n=1, it can be eliminated from the formula. This produces the much more commonly used formula λ γ= 2 N.A.

to describe the theoretical optical resolving power of a microscope. This formula does not, however, take account of the optical resolving power of a further optical system – the human eye. When both optical systems – microscope and eye – come into play, i.e., whenever the user looks into the microscope, the actual optical resolving power deteriorates somewhat. This is incorporated into the formula using a simple factor as follows:

1,22 λ λ γ= =0,61· 2 N.A. N.A.

In the case of white halogen light, which is typically used, an average wavelength of l=0.55 µm is generally applied, thus allowing simple calculation of the optical resolving power based on the numerical aperture indicated in the specifications.

b. Digital resolving power

A digital camera is frequently used for documentation purposes or for performing complex microscope evaluations. In this case, the resolving power of the camera, respectively the cam- era chip, must of course also be taken into account. This digital resolving power is, of course, dependent on the size and number of pixels of the chip used. It is not true, by the way, that a larger number of pixels always produces a better picture, since too many, too small pixels can quickly cause interfering flickers on the image. Unlike optical resolving power, it is not possible to indicate a minimum length, or a minimum recordable distance in micrometers or nanometers in the case of digital resolving power since a pixel can represent an area of the image measuring several tenths of a millimeter or just fractions of a micrometer, depending on the upstream magnification. As such, the only rule that applies is that the pixel count of a chip should ensure that the smallest detail of the work piece to be imaged should be represented by at least two, prefer- ably three pixels.

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PRE1305Optics glossary.indd 14 01.12.14 13:40 Eye position

The position of the eye in front of the eyepiece during the examination is called the eye posi- tion. Eyepieces can only display a good image if the eye is in this eye position. Many eyepieces have a high eye position, which means that the eye does not need to be pressed against the eyepiece but must be positioned slightly above it. This has the advantage of allowing people who wear spectacles to look through the eyepiece without first having to remove their spectacles. The disadvantage is the lack of clarity as to where the optimal eye position is, given the lack of contact. Such eyepieces therefore usually have eye cups. These are plastic cups that adapt to the face and shade off lateral scatter light while at the same time fixing the eye in the higher eye position.

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PRE1305Optics glossary.indd 15 01.12.14 13:40 Exit pupil

In the case of Koehler illumination, each image of the aperture diaphragm in the light path of the microscope is termed a pupil. The pupil located closest after the optical system being examined is called the exit pupil. It might help to remember that the eye position of an eyepiece is the exit pupil of a microscope – the pupil in the human eye must therefore exactly match the position of the exit pupil to see the best possible image. Since the pupil in the human eye bundles incident light on the retina, a focused image cannot be seen at the position of the pupil. On the contrary, the lack of focus is at its greatest at this point. In this position, the light of each pixel is fanned out across the diameter of the exit pupil and subsequently bundled back into one pixel by the pupil in the eye. This means that at the height of the exit pupil an area of light matching the size of the exit pupil diameter is shown instead of a single, clear pixel, which explains why the lack of focus is greatest. The exit pupil can, of course, be examined for just part of the microscope, for example, an objective lens also has an exit pupil. This objective lens exit pupil is needed, for example when designing a microscope, in order to be able to determine the maximum distance between objective lens and tube lens, up to which the image does not deteriorate in quality due to shadowing. The indication of the exit pupil diameter is also important if an objective lens is to be used to project a laser ray on to the work piece. The relatively high energy density of the laser ray can be reduced to pass through the objective lens by widening the light of the laser ray to the size of the exit pupil. The laser light passes through the objective lens at a lower energy density and re-bundles the ray precisely on a point on the work piece surface, thus again ensuring the required high energy density.

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PRE1305Optics glossary.indd 16 01.12.14 13:40 The calculation of the exit pupil diameter of an objective lens is dependent on the numerical aperture and focal length of the objective lens:

ø = 2 · (N.A.) · f

ø Image surface Exit pupil of the eyepiece

Image- Rear objective generating f2 (tube) lens focal plane ø2

Aperture Lens ø1 diaphragm f1

Specimen surface NA

Light source

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PRE1305Optics glossary.indd 17 01.12.14 13:40 Axial chromatic aberration

Lenses refract different wavelengths of light differently. Since different wavelengths manifest themselves as different colors to the human eye, white light is therefore split into individual colors by a lens. Blue light has a short wavelength and is therefore refracted much more strongly than red light, which has a long wavelength. In theory, light is bundled through a convergent lens in such a way that all light rays collect at one point – the focal point. In actual fact the light ray is, however, broken down into its individual colors, each of which has its own focal point. The focal point of blue light is closer to the lens – because of the stronger refraction – than that of red light. The individual focal points along the optical axis form a color gradient. This effect is called axial chromatic aberration. This aberration produces a color gradient on edges of a work piece, since if one light is focused and presents a sharp image of the edge, all other colors show the edge slightly out of focus. Axial chromatic aberration caused by objective lenses can be largely minimized by means of achromatic or apochromatic correction. Lens

Green focus White light

Lens Blue light

Red light

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PRE1305Optics glossary.indd 18 01.12.14 13:40 Field curvature

Field curvature is a geometric aberration and one of the five Seidel aberrations. Field curvature is the term used to describe the fact that a lens does not reproduce a flat subject as flat. If the center of the subject is sharply focused on the image, the peripheral areas are out of focus, and vice versa. This is caused by the spherical shape of the lens. Light rays emanating from a point to the side of the optical axis collect in a slightly different focal point than rays emitted from a point on the optical axis.In order to obtain a completely focused image, the plane of the image – such as a screen – would have to be curved. The curved retina in the human eye, for example, enables it to correct the field curvature.

Flat subject

Curved image

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PRE1305Optics glossary.indd 19 01.12.14 13:40 Image position / Intermediate image position

The image position indicates at which point along the system’s optical axis the image of the subject will be shown in sharp focus. Complex optical systems, such as microscopes, have several image positions and not just the one. The generated image is recaptured by a further component of the system and reproduced again in another position. Since there is no screen or comparable display medium in the position of the first image, it may be generated in this position, but we cannot see it. Which is why it is called an intermediate image, and its position the intermediate image position. The eyepiece of a microscope, for example, acts as a magnifying glass, thus further enlarging the intermediate image generated by the objective lens. If a camera is placed on a microscope, care must be taken to ensure that the element capturing the image – a digital chip or light-sensitive film – is placed exactly on an image position. This is why microscopes with camera output have a standardized interface that precisely defines the distance from the interface to the next image plane (C-Mount interface).

Intermediate image position Intermediate image / Intermediate image level Eyepiece Eyepiece

Intermediate image position

End position of the optical fiber Condenser

Objective lens (infi nitely Eye corrected) Zoom tube lens Tube lens Half mirror

Objective lens Reflector

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PRE1305Optics glossary.indd 20 01.12.14 13:40 Vision measurement

According to its definition, image processing is the “generation of digital images for subse- quent processing and evaluation”. Neither the means used to generate the digital image nor the approach adopted to subse- quently processing and evaluating the same are defined. The most frequent image processing applications are found in graphic image enhancement, e.g. photography. One typical application in this respect is the elimination of the “red eyes” caused by flashlight photography. There are, however, numerous other fields of application. One small field is the use of image processing in length measurement. In this case the information relating to the brightness of the image is used, for example, to enable the automatic detection of edges in the image. The pixel positions of the edges are subsequently translated into coordinates and lengths and offset against the machine coordinates.

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PRE1305Optics glossary.indd 21 01.12.14 13:40 Refraction

The angle of a ray of light changes slightly at the point where the light passes from one light- permeable medium to another, for example on the surface of water or the surface of a glass lens. This process is one of the oldest optical phenomena known to man, and is called (light) refraction. It is caused by the different speeds at which a ray of light can move in different materials. If you take two points in the different media and examine the path taken by the light from one point to the other, this path may not be the geometrically shortest route, given the bend, but it is the shortest, i.e., quickest route in terms of time, given the different speeds of mitigation.

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PRE1305Optics glossary.indd 22 01.12.14 13:40 Law of refraction

A ray of light refracts as soon as it passes from one light-permeable material to another, i.e. the angle of the ray changes slightly relative to the surface of the material transition. The direction and magnitude of this change in angle are dependent on the refraction indices of the two materials involved. The following applies: sin(α) · n1 = sin(β) · n2 where α = Angle of the incident ray β = Angle of the refracted ray n1, n2 = Refraction indices of the two materials

The law of refraction n = index of refraction

Refl ecting ray

α α Medium 1 Boundary layer n1 Medium 2 n2 α sin β n2 β Refracted ray

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PRE1305Optics glossary.indd 23 01.12.14 13:40 Refraction index

The refraction index is a material constant relating to optically active, i.e., light-permeable materials. It is calculated on the basis of the ratio of the speed of light in a vacuum to the speed of light within the relevant material. The higher the refraction index, the slower the light moves in the medium. Given the principle of fastest light mitigation, refraction becomes more pronounced the larger the difference is between the refraction indices of the two materials through which the light passes. As the vacuum serves as the reference medium, it has an exact index value of 1. Further typical refraction indices include:

Material Index Material Index Vacuum 1.000000 (Reference) Sodium chloride (common salt) 1 1.544 Alcohol 1.329 Sodium chloride (common salt) 2 1.644 Amorphous selenium 2.92 Polystyrene (styrofoam) 1.55 Acetone 1.36 Quartz 1 1.644 Chromium oxide 2.705 Quartz 2 1.553 Diamond 2.417 Quartz glass 1.46 Ice 1.309 Ruby 1.77 Ethanol (methylated spirits) 1.36 Sapphire 1.77 Fluorite (Fluorspar) 1.434 Carbon disulfide 1.63 Liquid carbon dioxide 1.2 Heavy flint glass 1.65 Glass 1.5 Heaviest flint glass 1.89 Methyl iodide 3.34 Light flint glass 1.575 Calcite 1 1.66 Emerald 1.57 Calcite 2 1.486 Topaz 1.61 Crystal 2 Water (20°C) 1.333 Crown glass 1.52 Zinc crown glass 1.517 Copper oxide 2.705 Sugar solution (30%) 1.38 Lapis lazuli 1.61 Sugar solution (80%) 1.49 Air (close to the ground) 1.00029

* Diamonds have the highest refraction index of all natural materials.

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PRE1305Optics glossary.indd 24 01.12.14 13:40 CCD (Charged Coupled Device)

Charged coupled device is the term used to describe a silicon chip with a grid pattern of tiny light-sensitive diodes (photodiodes). Each photodiode gathers information relating to the amount of light that touches a specific part of the CCD chip during the exposure period. This brightness information is then sequen- tially selected and converted into digital information. The simplest form of conversion is the classification of luminance in 256 classes of equal size. Each collected luminance of each individual pixel can therefore be assigned to a class between 0 and 255. The larger the col- lected quantity of light, the larger is also the classification number. If 0 is taken as absolute black and 255 as absolute white, then a specific shade of gray can be assigned to each class between 0 and 255. If the pixels are then displayed on the screen using the same grid pattern as the chip with its corresponding shades of gray, a digital black/white image emerges.

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PRE1305Optics glossary.indd 25 01.12.14 13:40 Chromatic aberration

Chromatic aberrations are imaging errors caused by the different strengths of refraction of wavelengths of light. The refraction of short-wave light rays is more pronounced than that of long-wave rays. Since the individual wavelengths manifest themselves as different colors, one might also say that colors demonstrate different levels of refraction. This is why aberra- tions caused by this phenomenon are called chromatic (chromatic = relating to color [Greek]). A distinction is made between two chromatic aberrations - axial chromatic aberration and color magnification error.

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PRE1305Optics glossary.indd 26 01.12.14 13:40 CMOS (Complementary Metal Oxide Semiconductor)

CMOS chips are digital photo chips with a grid pattern of tiny, light-sensitive photodiodes. During exposure, these pixels collect information relating to the amount of light touching a specific point on the chip and convert this information straight into an electronic voltage signal. In addition to the light-sensitive sensors, the chip also houses conductors and controllers that can be used to read out the brightness of any pixel at any time. Special CMOS chips also offer color information by layering three photodiodes one beneath the other for each pixel. Since long-wave red light penetrates a silicon crystal considerably deeper than short-wave blue light, the color information (blue, green, red) can also be assigned to each layer, in addition to the information relating to the brightness.

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PRE1305Optics glossary.indd 27 01.12.14 13:40 C-Mount

When examining a subject through a microscope, there is only one specific point along the optical axis of the system at which a sharply focused image is generated. When using cameras – regardless of which type – it is therefore eminently important to position the element gener- ating the image – be it a digital chip or light-sensitive film – at precisely this point. Microscopes and cameras are, however, generally manufactured by different companies, and to this day cameras are generally not built into microscopes. For this reason, the interface between the two systems has been standardized in order to ensure the largest possible number of combinations of microscopes and cameras. This C-Mount interface substantially consists of a simple 1” thread for mounting the camera onto the microscope. The exact positioning of this thread relative to the image position is, however, precisely defined. So, if a microscope has a C-Mount interface for camera output, the image is guaranteed to be in sharp focus at precisely 17.52 mm above the mounting flange of the thread.

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PRE1305Optics glossary.indd 28 01.12.14 13:40 Deviating mirror, Deviating prism

This special type of mirror or prism has two mirror or boundary surfaces that are generally arranged at right angles to each other. If the surfaces are arranged in a specific way, the image is inverted without changing the optical path.Deviating prisms, for example, are used in top quality profile projectors in order to rotate the projected image, thus presenting a true to side image instead of the usual inverted image shown by standard projectors.

Dichroic mirror

A dichroic mirror is a special type of semi-permeable mirror. It only reflects a specific wave- length or color while being absolutely permeable for all other wavelengths or colors. A dichroic mirror can be used for exact extraction or combinations of specific colors.

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PRE1305Optics glossary.indd 29 01.12.14 13:40 Differential interference contrast

Commonly abbreviated to DIC. This method of analysis aims to make even the smallest differences in surface height clearly visible in the image – even if these height differences are much smaller than the depth of field range of the objective lens, i.e. cannot normally be detected. This is achieved using a special type of illumination. The cold halogen light is first polarized to create synchronous waves. These are the only waves that can subsequently generate interferences by means of overlap- ping. The polarized light is then channeled through a special prism. This DIC prism (also known as a Nomarski prism or Wollaston prism) actually consists of two overlaying prisms. The material of the first prism separates the light into two rays. This means that two slightly offset refracted light rays pass through the prism, instead of just the one. These two beam paths are called ordinary and extraordinary rays, since one path corresponds to the law of refrac- tion – so it is ordinary – whereas the second runs at a slightly different angle and is therefore extraordinary. The second prism breaks both rays so that they touch the work piece at parallel points a small distance apart. The mismatch between the ordinary and extraordinary rays depends on how long the rays run through the prism before being aligned in parallel by the second prism. The longer the path, the stronger is the difference in angle between the rays and, consequently, the farther is their subsequent parallel path. A lateral adjustment of the DIC prism can there- fore alter the mismatch of the rays and thus ensure the best possible setting for the relevant application. Farther along, the parallel light rays touch the work piece, are reflected by the surface and touch the DIC prism again, which then re-combines the rays into a single beam. In doing so, the light waves from the reflected ordinary and reflected extraordinary rays are aggregated. If the height of the work piece differs in the range between the two light rays, one of the rays will be slightly longer than the other.

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PRE1305Optics glossary.indd 30 01.12.14 13:40 As such, when overlapped, the waves are no longer perfectly synchronized but, instead, slightly offset from each other. If these offset waves are aggregated, they create a new wave with a different wavelength. And since a different wavelength automatically equates to a different color in the case of light, a different color emerges as a result of the overlapping. Thus, even tiniest differences in height on the surface of the work piece can be made imme- diately visible since they appear in a different color in the image. Different pre-defined heights can then be assigned to each color. The medical and biological sectors frequently use DIC in transmittent light, for which two DIC prisms are required – one to separate and one to re-combine the light rays. Since specimens observed in transmittent light have to be very thin, however, in order to generate the interfer- ence effects described above, this type of application is very rare in industrial use.

Intermediate image

Tube lens Analyzer

Polarizer Light source Beam splitter

DIC prism

Objective lens

Ordinary ray Extraordinary ray

Work piece

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PRE1305Optics glossary.indd 31 01.12.14 13:40 Dual image

Undesired internal reflections on the optical components of the microscope can cause the generation of a second, slightly offset image in addition to the actual image. Since the images overlay each other, this phenomenon is called a dual image. A dual image is neither out of focus, nor shadowed; it is just double.

Dual refraction

When a light ray touches a joint between one light-permeable medium and another, the light ray is refracted, i.e., it changes direction. In the case of many crystals, e.g. calcite, the light ray is not refracted just once, but twice, with the individual refracted beams running at slight differences in angle to each other.

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PRE1305Optics glossary.indd 32 01.12.14 13:40 Dark field analysis

This method of analyzing incident light ensures that no light touches the work piece through the objective lens. Instead, the light is channeled through a ring-shaped lens placed around the actual objective lens. The lens bundles the light in the objective lens’s field. The light is, however, substantially reflected past the objective lens by the surface as a result of the slanted angle of incidence. Only flaws on the surface, such as scratches or soiling, scatter the light, so that some of the light also touches the objective lens.The result is a very dark image of the work piece surface, on which only flaws are brightly illuminated and therefore very easy to identify.

Tube lens

Beam splitter

Objective lens

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PRE1305Optics glossary.indd 33 01.12.14 13:40 Transmittent light

In the case of transmittent light, the work piece or specimen to be examined is placed between the light source and the objective lens. This ensures that edges on thicker work pieces are shown at optimum contrast, since the light is shaded off by the work piece but can enter the objective lens unhindered alongside the work piece. This strong contrast can be used to facili- tate and raise the certainty of distance or diameter measurements, for example. There is a danger of irradiation when using transmittent light. If the light source is too bright, the light shines over the edge into what should actually be the shaded area of the work piece. As a result, transitions that are actually focused go out of focus or – which is even worse when performing measurements – the edge merges into the work piece and is therefore incorrectly measured. Diffraction has a similar effect, e.g. on cylindrical work pieces that are laid flat. The use of an aperture diaphragm can, however, prevent this effect or at least reduce it to a level that is irrelevant for measurement purposes. In the case of very thin specimens, such as are common in medicine or biology, the light also penetrates the specimen – more easily at thin points than in thicker areas. This makes cell structures, for example, very visible.

0 20 = f 0.42

0 I O BD PlanO Apo 20

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PRE1305Optics glossary.indd 34 01.12.14 13:40 Built-in microscope

Built-in microscopes are microscopes that are reduced to the optical system – without tripod or specimen table. On their own, they are equipped with one light source at the most, since coaxial incident light is transmitted through the objective lens onto the work piece and therefore has to be channeled into the microscope’s optical system. Built-in microscopes are incorporated into machinery and equipment when a microscopic examination of the result is required between the individual stages of production or if micros- copy itself is needed for the production process. This is especially the case in the semiconductor industry. Some built-in microscopes do not even have an eyepiece. Instead a camera is mounted for viewing purposes and the images it generates can even be automatically evaluated and ana- lyzed with the help of a computer.

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PRE1305Optics glossary.indd 35 01.12.14 13:40 Entrance pupil

The aperture diaphragm is imaged several times over the course of the microscope's light path. These images are called pupils. Accordingly, the final pupil before the objective lens is called the entrance pupil.

Eclipse / Shadowing / Vignetting

Eclipse is the term used in astronomy to describe the occlusion of one celestial body by another, e.g. an eclipse of the sun or moon.In the field of optics, the term is used to describe a slightly different phenomenon – the partial darkening at the edges of the field.This shadowing effect occurs when interference arises during the transmission of optical infor-mation by the objec- tive lens and eyepiece or if examinations are performed without using a field diaphragm.

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PRE1305Optics glossary.indd 36 01.12.14 13:40 Finitely corrected optical system

Finitely corrected optical system is the term always used when only one objective lens or one lens is used to generate images. In the case of finitely corrected microscopes, the distances from the objective lens to the work piece and to the intermediate image is precisely defined. Neither of these distances can or may be altered without at the same time adjusting the magnification factor. Finitely corrected systems are particularly cheap since they do not contain any elaborate internal optical components. On the other hand, their use is very limited, given the fixed distances. Moreover, additional filters or beam splitters can quickly cause unwanted dual images. 2 L 1 L

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PRE1305Optics glossary.indd 37 01.12.14 13:40 Color temperature

The color temperature can be used to proportionately compare the light coloring of any light source relative to the color of natural light (sunlight). “Colored” lighting acts as a color filter, causing corresponding changes to the image. For example, yellowish lighting cannot distin- guish between blue and black, which can result, in particular, in unpleasant surprises when buying clothes in shops with such illumination. The color temperature scale is based on an absolutely black subject that is slowly heated. The color changes depending on the temperature. At first, the subject appears in dark red. As the temperature increases, the shade of red becomes lighter and lighter and gradually changes to yellow. This is followed by white light, which eventually turns blue at even higher temperatures. The color of natural light corresponds to a color temperature of 5500 K on this scale, i.e., white. By contrast, the halogen lamps used in microscopes generally only have a color tem- perature of about 3200 K. The use of suitable filters can, however, simulate an increase in color temperature and thus create virtually natural illumination, especially for documentation purposes.

Color temperature scale

10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 2.000 1.000

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PRE1305Optics glossary.indd 38 01.12.14 13:40 Color magnification error

This imaging error belongs to the group of chromatic aberrations. The different degrees of refraction of the individual wavelengths or light colors and the ensuing difference in focus points cause a subject to be magnified to differing levels depending on the color. This is visual- ized by color fringing on the edges of the image.

Blue Red

Field diaphragm

In the microscope’s optical system, the positioning of the field diaphragm – which is also known as the view field diaphragm - ensures that an image of the diaphragm lies precisely on the subject plane and, as such, on the image plane. This reduces the microscope’s field, cuts out shadowed edges of the image, and masks interfering lateral scatter light. The result is a clear image of the subject that is lit at even brightness across the entire visible area.

Diaphragm Objective lens Subject Subject plane plane

Condenser

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PRE1305Optics glossary.indd 39 01.12.14 13:40 Flickering

Unlike the usual meaning of the word, lens or objective lens flicker does not describe any flickering in the image but rather unwanted light rays superimposed on the image. Some of the light is reflected rather than refracted at each lens in an optical system. If the light reflected by one lens touches a different upstream lens, part of the reflected light is reflected again, and influences the quality of the image. Such a reflection does not only occur when light enters a lens, but also when it exits, which can then even cause total reflection. Ultimately, the ensuing image becomes paler and more opaque as a result of the aggregate of all internally reflected light rays. Flickering can be countered by means of anti-reflection coating on each individual lens in the system, such as is used for spectacle lenses in order to reduce irritating reflections of light on the lenses. Such anti-reflection coating can dramatically reduce the level of flicker – in some cases by as much as 80%!

Fluorescence ring light

Fluorescence ring light is a special type of ring light whereby the ring-shaped light source generates a light that largely corresponds to natural light. This enables the generation and documentation of images with very good color fidelity.

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PRE1305Optics glossary.indd 40 01.12.14 13:40 Fluorescence analysis

Light is generally absorbed, at least to a minor proportion, and translated into heat when it touches a subject. This is how objects gain their coloring, since some wavelengths of white sunlight are absorbed, whereas others are reflected. Some materials do not, however, convert the absorbed light into heat and instead re-emit a light with a larger wavelength following absorption. This well-known process is called fluo- rescence. Fluorescence analysis focuses on the fluorescent behavior of materials. It is used in medicine and biology, in particular, to analyze cells and requires in-depth expert knowledge.

Fluorescence microscope

Fluorescence microscopes are designed specifically for fluorescence analysis purposes. On the one hand, they are fitted with a particularly intensive light source, such as a xenon lamp, and on the other hand with filters and dichroic mirrors to filter exactly those wavelengths out of the image that are emitted by the light source. Only waves with longer lengths emitted by fluorescent material can penetrate the filters and generate the fluorescence image.

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PRE1305Optics glossary.indd 41 01.12.14 13:40 Focus, focus point / Focal point

Generally speaking, a subject is deemed to be in focus when the sharpness of the image generated by the microscope is optimal, or focused. There is, however, a second meaning to the term. Focus point, focal point or even focus means the point at which parallel rays of light are bundled by the lens. The focus point must be located between the subject and the lens if the lens is to show the subject.

Focal width / Focal length

Roughly speaking, the focal width or length is the distances between the lens and the focus point. In utterly correct terms, however, it is the distance between the relevant principal plane of the lens and the focus point.

S2 S1 F2 F1 G B

f f' g b H1 H2 f: Focal width / length on the subject side f’: Focal width / length on the image side g: Subject width b: Image width G: Subject size B: Image size

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PRE1305Optics glossary.indd 42 01.12.14 13:40 Galileian type stereo microscope / Parallel stereo microscope

The light paths for both eyes are separate, but nevertheless largely parallel in the case of the Galileian stereo microscope. Only one objective lens is used; the light paths are emitted from the lateral areas of the objective lens. The light emitted by the subject that passes through these peripheral areas has an angle of about 5-7°. As two light paths are emitted from two sides of the objective lens, these have an ideal viewing angle of about 10-14° for three-dimen- sional viewing. The two light paths continue, however, in parallel between the objective lens and the eyepiece, and not at angles, which enables a semi-permeable mirror to be used, e.g. to obtain an additional camera output. Changing the length between objective lens and eyepiece to suit the relevant circumstances is also possible without problems. The disadvantage of this process is the comparatively expensive objective lens. In order to ensure the best possible imaging quality, the quality of the objective lens, especially its peripheral areas, must be the best possible – since the light paths are emitted from here to the eyepiece.

Multiple (Ghost) image

A ghost image emerges when a second incorrect image of the work piece is generated in a position other than the image position (i.e. upstream or downstream) that then overlays the actual image like a shadow.

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PRE1305Optics glossary.indd 43 01.12.14 13:40 Total magnification

A microscope is comprised of several optical sub-systems, each of which has its own magnifica- tion or own reproduction scale. If an optical system generates a virtual image – as is the case, e.g., with eyepieces – the image is said to be magnified. By contrast, systems such as objective lenses or tube lenses generate a real image (as intermediate image). In this case, this is known as a – measurable – reproduction scale. The combination of all optical systems produces a multiplication of the individual magnifica- tions and reproduction scales to form the so-called total magnification of the microscope. The total magnification of a microscope is 100x, for example, if the reproduction scale of the objective lens is 10x and the magnification of the eyepiece is equally 10x. The same total magnification can be achieved by using a 5x objective lens and a 20x eyepiece. This solution is considerably cheaper but the quality of the image is substantially poorer, since the objective lens alone determines the resolving power and objective lenses with a higher reproduction scale generally also have a higher numerical aperture.

Greenough stereo microscope

In the case of Greenough stereo microscopes, the light paths for each eye are completely separate from each other along the entire path at an angle of 10-14°. This is a very cheap design albeit with severe restrictions caused by the ever increasing divergence of the light paths and the use of a finitely corrected optical system. Additional equipment, such as an additional camera output, is very difficult, if not impossible to integrate into such a system.

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PRE1305Optics glossary.indd 44 01.12.14 13:40 Green interference filter (GIF)

Interference filters are only permeable to light rays in a small and relatively clearly defined range of wavelengths, smaller or larger wavelengths are eliminated by interference effects. Such a filter consists of several layers of thin film mounted on a substrate. The partial rays of light reflected and transmitted on the boundary layers between the individual layers overlay and interfere, i.e., the waves aggregate. The choice of thickness of the individual layers is designed to ensure that waves of a certain length aggregate to precisely zero, i.e., are eliminated. The benefits of interference filters as opposed to absorption filters, which absorb all undesired wavelengths, are the very clearly defined boundaries between transmitted and filtered wave- lengths, together with the low level of heat generation. Since absorption filters convert the absorbed light energy into heat, they quickly heat up and can only be used at relatively low light intensity. Interference filters can be used for powerful lighting and, depending on the application, can even strengthen the contrast and resolving power of the optical system. Green interference filters are only permeable to green light. The central wavelength is 570 nm, with a half width of 70 nm, which means that most light rays with a wavelength of 570 nm are transmitted, whereas wavelengths of just 35 nm above or below this central value are only transmitted at half strength.

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PRE1305Optics glossary.indd 45 01.12.14 13:40 Half mirror / Semi-permeable mirror / Beam splitter

A semi-permeable mirror consists of a sheet of glass on which a thin coating – usually metal – is applied to one side. This ensures that light touching the mirror is partially reflected, but also partially transmitted. Although half mirror or semi-permeable mirror have become commonly accepted terms, they are not necessarily quite correct. The type of coating determines whether more or less light is reflected or transmitted; the ratio is not always 50:50. As such, beam splitter is a better term for describing such optical components.

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PRE1305Optics glossary.indd 46 01.12.14 13:40 Principal point / Principal plane

A ray of light that passes through the focal point of a spherical lens is refracted twice by the lens – when it enters and when it exits. It subsequently runs parallel to the optical axis. If the entering and exiting rays are extended to a point where they intersect (in the lens), and if the same procedure is applied to all rays passing through the focal point of the lens, the intersections produce a plane, the so-called principal plane. The point of intersection between the principal plane and the optical axis is called the principal point and defines the position of the principal plane. Rays of light running parallel to the optical axis are also refracted twice by the same lens, and merge into one focal point behind the lens. If the entering and exiting rays are extended in this case, their points of intersection also form a plane. In the case of thicker lenses, this is not, however, identical to the principal plane described above. In such cases, a distinction is made between the principal plane on the subject side and the principal plane on the image side, depending on which plane is closer to the subject or closer to the image of the same.

S2 S1 F2 F1

F1: Focal point / Focus point on the subject side F2: Focal point / Focus point on the image side H1 H2 H1: Principal plane on the subject side H2: Principal plane on the image side S1: Apex on the subject side S2: Apex on the image side

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PRE1305Optics glossary.indd 47 01.12.14 13:40 Principal ray

Principal ray is the term used to describe the beam of light that passes exactly through the center of the entrance pupil of an optical system. It therefore generally defines the ray of light that is directly on the optical axis and therefore reflects the principal light path within the optical system.

Intermediate image position

Eyepiece

Intermediate image position

End position of the optical fiber Condenser

Zoom tube lens

Half mirror

Objective lens Reflector

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PRE1305Optics glossary.indd 48 01.12.14 13:40 Bright field analysis

This type of analysis involves shining light onto the visible area of the work piece to ensure that it is brightly lit and reflects light to the objective lens in order to generate the image. As such, bright field analysis is the classic approach using incident light, whereby it is irrelevant whether the light is first channeled coaxially through the objective lens or emitted by a ring light positioned around the objective lens.

Bright fi eld analysis Tube lens

Beam splitter

Objective lens

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PRE1305Optics glossary.indd 49 01.12.14 13:40 Infrared microscope

Infrared microscopes enable the examination of work pieces using light in the infrared wave- length range. To this end, they are fitted with an infrared light source and a special camera that helps to make the infrared light visible. Examinations using infrared light are particularly common in the semiconductor industry. Note, however, that just mounting an infrared light source and infrared camera onto a clas- sic microscope is generally not sufficient to allow this type of examination. Microscopes, and especially objective lenses, that are not designed for this application are only permeable to infrared wavelengths to a very limited extent and would therefore lose an enormous portion of their performance capabilities. Moreover, infrared light can cause serious damage if it touches the eye through the eyepiece without protection. By contrast, infrared microscopes only let infrared light through when the light path to the eyepiece is completely closed by a shutter, i.e. closing mechanism. Special infrared objective lenses compensate chromatic aberration even in the infrared wavelength spectrum, i.e., the workpiece can first be optimally focused using visible light before examining it with infrared light – without the need to re-focus.

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PRE1305Optics glossary.indd 50 01.12.14 13:40 Interference

When waves overlap, they influence each other to the extent that their amplitudes are aggre- gated. In the case of two synchronous waves, this means that the amplitude doubles, whereas if a wave is offset by a half wavelength, the amplitudes aggregate to zero at each point, i.e. the waves are eliminated. This reciprocal influence is known as interference. Since rays of light are electromagnetic waves, such interferences also occur in optics. Light waves only interfere, however, when they oscillate in the same direction. This can be achieved, for example, by means of polarization.

Inverted microscope

Since inverting more or less means turning upside down, an inverted microscope is nothing more than a upside down microscope. This basically means that the table supporting the subject is located above the microscope optics, meaning that you examine the subject through the microscope from below. One of the major advantages of this design over conventional microscopes is obvious: The microscope can even be used to examine work pieces that, by dint of their height, will not fit between the table and the microscope itself. A much bigger advantage, however, is the fact that the design ensures that work pieces are always exactly in focus when they are placed on the table. The subject plane is always identical to the table plane – eliminating the need for any time-consuming re-focusing.

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PRE1305Optics glossary.indd 51 01.12.14 13:40 Coaxial incident light

In the case of coaxial incident light, diffuse light is first bundled and channeled via a beam splitter into the light path of a microscope in such a way that the center of the light bundle runs along the optical axis of the microscope, i.e., both light paths run coaxially. The condenser lens used to bundle the light ray generates a magnified image of the light source, respectively, the aperture diaphragm exactly on the exit pupil of the microscope’s objective lens. This ensures that the light emitted by the objective lens illuminates the entire visible area evenly.

Tube lens

Beam splitter

Objective lens

Koehler illumination

The special design of the optimized microscope illumination developed by August Köhler (1866-1948) ensures even illumination of the image across the entire visible area with strong contrasts and high optical resolving power. This is achieved by means of a special light path, which passes from the light source first through a collector lens, then through two diaphragms – the field and aperture diaphragms – and finally through a condenser lens, before illuminating the work piece.

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PRE1305Optics glossary.indd 52 01.12.14 13:40 Collimator (lens)

A collimator is an optical component for generating parallel light. The simplest type of collima- tor consists of a perforated diaphragm in the focal point of a lens or lens system.

B L

Coma

Coma is the term used to describe a geometric aberration and is one of the 5 Seidel aber- rations. A coma occurs when a point located relatively wide of the optical axis is imaged. Because of the steep angle of incidence, light emitting from this point is not bundled back into one point on the lens. Instead, the various ring zones of the lens produce different images of the point. The farther the light touches toward the outside of the lens, where it is more strongly curved, the larger becomes the image of the point and the closer the image is located to the optical axis. Overall, the coma causes the point to be shown as a point with a tail of light that becomes increasing weaker and broader. This is why the term “Coma” is used for this aberration as the Latin word coma means comet.

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PRE1305Optics glossary.indd 53 01.12.14 13:40 Condenser (lens)

A condenser is an optical system for optimizing the illumination of a microscope. It ensures that, ideally, all of the light from the work piece touches the objective lens, thus producing a bright image. A condenser also increases the optical resolving power of a micro- scope by ensuring that the opening of the aperture diaphragm – which is responsible for the resolving power – is completely and evenly lit.

Intermediate image position

Eyepiece

Intermediate image position

End position of the optical fiber Condenser

Zoom tube lens

Half mirror

Objective lens Reflector

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PRE1305Optics glossary.indd 54 01.12.14 13:40 Confocal system / Confocal microscope

Confocal optical systems are used when the image needs to demonstrate extremely good resolution in focal direction, i.e., when clear information relating to the height of the work piece is required rather than the usual microscopic image. The confocal principle involves shining the light from a point-shaped light source from behind onto the objective lens, which then images the same – again, as a point – on the surface being analyzed. Since this light path can be reversed, this means in turn that all of the light emitted or reflected by this one point on the surface is precisely imaged by the objective lens on the light source again. A beam splitter can be used to divert this reflected light path and direct it on to a very small perforated diaphragm whose positioning corresponds exactly to that of the light source. The point of light is imaged precisely in the center of the perforated diaphragm – i.e., the light passes through the diaphragm - only when the work piece is exactly in focus, i.e., when the light source is imaged precisely on, and reflected by the surface. If the work piece is out of focus or if other points apart from the specific point of the surface emit scatter light, this light is masked virtually completely by the perforated diaphragm. So if a light-sensitive sensor, such as a photodiode, is mounted behind the perforated diaphragm, sharp deflection of the light is only possible if the surface is optimally focused. This produces very accurate information relat- ing to the height of a surface point at the point where the optical axis of the confocal system penetrates the surface. Confocal microscopes must be fitted with high resolution scales on all three axes in order to generate an image of the surface. Because, initially, the confocal system only provides informa- tion relating to the focus position at one point. By means of a grid – i.e., by capturing numer- ous height readings in a close-knit grid of numerous adjacent points – the three-dimensional image of the work piece surface can be generated with the help of a computer.

Point-shaped light source Subject

Subject too close Detector Subject in focus Subject too far away

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PRE1305Optics glossary.indd 55 01.12.14 13:40 Laser

The term laser is actually an acronym, and stands for Light Amplification by Stimulated Emission of Radiation. Nowadays, however, the term is also used to describe the actual light sources or equipment used to generate a beam of laser light. A monochromatic (single-colored), coherent (unidirectional) and parallel bundle of rays of a, generally, very small diameter is generated by a laser. Lasers are categorized by the materials used to generate the light. Depending on the properties of these materials, a distinction is made between solid-state lasers, liquid lasers, gas lasers or semiconductor lasers (laser diodes). Furthermore, a distinction is made between pulsed lasers, which emit light in short impulses or flashes, and continuous wave lasers, which, by contrast, generate a consistent laser beam over time. The best-known types of laser include the ruby laser, the YAG laser (yttrium aluminum garnet) and the HeNe laser (helium neon).

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PRE1305Optics glossary.indd 56 01.12.14 13:40 LB filter

Light balance filters are used to simulate natural light illumination on a microscope and to largely eliminate color errors on the image. The halogen light sources used on microscopes have a lower color temperature than natural light, i.e., the light is more yellow or reddish depending on the extent to which the light source is dimmed in order to achieve the desired light intensity for good analysis. The use of an LB filter for this yellowish-reddish light, however, filters out the long-wave red light waves, thus generating a light whose color more closely resembles natural light. This does, however, of course result in a greater loss of light intensity.

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PRE1305Optics glossary.indd 57 01.12.14 13:40 LED lighting

Light emitting diodes can be used instead of the halogen light typically used to illuminate microscopes. They have a much longer service life, consume much less energy, develop scarcely any heat, are very compact, and emit a clear, bright light. Their light intensity is, however, considerably weaker than is the case with halogen lamps. LED illumination is frequently insufficient for analysis at large magnifications, in particular. LEDs are available in various colors (red, white, green, blue ...). As such the color of illumination can be selected easily, without having to use special color filters.

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PRE1305Optics glossary.indd 58 01.12.14 13:40 Light

The nature of light is probably one of the most complex issues facing modern-day science.On the one hand, light demonstrates the properties of a body or particle, while on the other hand also demonstrating the properties of a wave. Explanations for this wave-particle dualism are very difficult; corresponding models have only been available since the beginning of the last century. Light dissipates as an electromagnetic wave at huge speeds (at 300,000 km/s in a vacuum). The human eye can only see waves within a relatively small range of wavelengths, between about 380-780 nm.

Microwaves and radio waves 10 km

Infrared rays

Visible light

UV rays 200 nm X-rays 10 nm Ionizing rays

Cosmic rays

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PRE1305Optics glossary.indd 59 01.12.14 13:40 Light microscope

Light microscope is the correct term for what is usually just called a microscope. A light micro- scope uses visible light to view a work piece or specimen at large magnification. Light micro- scopes always have an objective lens and an eyepiece that are used to view the work piece. The term microscope is not sufficient as a description since there are numerous technologies that can be used to view a subject at magnification, but which do not work with light - e.g., scanning electron microscopes, atomic force microscopes, etc.

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PRE1305Optics glossary.indd 60 01.12.14 13:40 Wavelengths of light

Light dissipates as an electromagnetic wave whose oscillations run perpendicular to the direc- tion of dissipation. The shape of these oscillations is an exact sinus, which makes it possible to distinguish between different waves based on their length. The wavelength is the distance at which the light dissipates during a complete oscillation.

Wavelength Long wave

Time unit Wavelength Short wave

Ultraviolet Infrared

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm

Cosmic Gamma X-radiation Ultraviolet Infrared Terahertz Radar Radio TV Medium wave radiation radiation radiation radiation radiation Long wave Short wave

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PRE1305Optics glossary.indd 61 01.12.14 13:40 Measuring microscope

Measuring microscopes are microscopes fitted with a measuring table, i.e. a movable subject table whose position is captured and known at all times with the help of two scales. The position of the table is represented in coordinate form using a digital display. Distances or diameters can be examined by setting the table to zero or adjusting its position. Even more complex measuring tasks can be performed with the help of additional evaluation units or computer programs. In addition, the third axis can also be fitted with a scale, thus enabling heights to be measured by means of focusing. In keeping with the tasks they are required to perform, measuring microscopes generally offer relatively little magnification compared with other microscopes.

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PRE1305Optics glossary.indd 62 01.12.14 13:40 Metallurgical microscope

Metallurgical microscopes are optimized for analyzing the surfaces of metal work pieces, such as silicon semiconductors. Analyses are generally conducted using incident light. In addi- tion to the typical bright field analysis, metallurgical microscopes also offer other viewing options, such as dark field analysis, viewing with polarized light or differential interfer- ence analysis. Metallurgical microscopes are generally also capable of working at very large magnifications (up to 4000x) and, accordingly, with excellent optical resolving power.

Bright fi eld analysis Dark fi eld analysis

Differential Polarization interference analysis

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PRE1305Optics glossary.indd 63 01.12.14 13:40 Near-infrared radiation

The human eye can detect electromagnetic waves at wavelengths of between 380 nm and about 780 nm as light in different colors. The longest visible waves generate a red light. If the waves become even longer, the radiation can no longer be recognized as a color, but is perceived as heat radiation by the human body. The waves of this infrared radiation can reach lengths of up to about 1 mm. A range of 780 nm to about 2500 nm, which is still very close to the range of visible light, is known as the near-infrared range. Special near-infrared objective lenses can be used to transfer wavelengths of up to about 1800 nm – at corrected focus, i.e., there is no need to re-focus between viewing with visible light and viewing with near-infrared light.

Ultraviolet Infrared

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm

Cosmic Gamma X-radiation Ultraviolet Infrared Terahertz Radar Radio TV Medium wave radiation radiation radiation radiation radiation Long wave Short wave

Near-ultraviolet radiation

Electromagnetic wavelengths that are slightly shorter than those of visible light are known as ultraviolet radiation. The ultraviolet radiation range begins at a wavelength of about 400 nm and less. The range between 290 nm and 400 nm is called the near-ultraviolet range. Special objective lenses with identical focus for visible light and radiation in the near- ultraviolet range are called near-ultraviolet objective lenses.

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PRE1305Optics glossary.indd 64 01.12.14 13:40 ND filter

ND stands for Neutral Density. ND filters only reduce the quantity of light passing through the filter. Light is reduced evenly, irrespective of the wavelength, no color changes occur. The quantity of light can be simply and easily reduced by reducing the voltage at the light source. This, however, also produces a change in color temperature, i.e., the light becomes considerably redder. ND filters are therefore used to take advantage of the high color tempera- ture – close to natural light – while at the same time not overexposing the work piece to the extent that details are not longer detectible. ND filters offer differing levels of light reduction. The level of reduction is indicated by an index number. An ND2 filter, for examples, reduces the light quantity to half, an ND8 filter even down to one eighth of the original quantity.

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PRE1305Optics glossary.indd 65 01.12.14 13:40 Numerical aperture

The numerical aperture is an important indicator of an objective lens. The dimensionless figure determines the resolving power and depth of field and influences the brightness of the image. The numerical aperture on typical light microscopes has a value of between zero and one. This figure can, however, be higher, if immersion objective lenses are used. A higher numerical aperture produces less and therefore improved optical resolving power. The depth of field is also reduced, producing better height measurement results when using measuring microscopes or vision measuring equipment, in particular. The calculation of the numerical aperture is dependent on the medium between the subject and the objective lens and on the angle aperture of the objective lens, i.e., the gradient angle of the ball of light that can just still be captured by the objective lens when emitted from a point. The ensuing formula for the numerical aperture is as follows:

N.A. = n · sin σ

The letter n stands for the refraction index of the medium between subject and objective lens. This value is generally 1, since the medium is air. The value is higher in the case of the aforementioned immersion objective lenses since the subject and objective lens are separated by immersion oil with a larger optical density.

P.D. P.D. W.D: Working Distance P.D: Parfocal Distance  W.D W.D σ: Half objective beam width

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PRE1305Optics glossary.indd 66 01.12.14 13:40 Nomarski prism

A Nomarski prism is a special prism needed for differential interference contrast analysis. This prism is also commonly known as a Wollaston prism or DIC prism (DIC = Differential Interference Contrast).

Analyzer

Second modified Nomarski prism

Objective focal plane

Objective lens

Objective plane

Condenser

First modified Nomarski prism

Polarizer

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PRE1305Optics glossary.indd 67 01.12.14 13:40 Subject-image distance

The total distance based on the work piece surface in focus on the one side of the optical system up to the position of the image on the other side is called the subject-image distance. This distance does not take account of any deflection of the light path, i.e. the distance is always measured along the optical axis and not across the space, which might be required for the final design. The subject-image distance is clearly and unalterably fixed in the case of finitely corrected objective lenses. Consideration of a brief bend in the optical axis is therefore imperative when incorporating a beam splitter in the design. By contrast, infinitely corrected objective lenses do not have a clearly defined subject-image distance since the light rays run parallel between the objective lens and the tube lens and this design can therefore be varied accordingly. There are, however, limits to the maximum distance – depending on the size of the required visible area – that may not be exceeded. Theoretically, at least, there are no minimum limits that must be observed. Although the tube lens will probably never be placed directly next to the objective lens if only for reasons of design. As such, the upper distance limit plays a much more important role for designers of optical systems.

Magnified image Objective lens Subject

Subject-image distance

Objective lens Tube lens Magnified image Subject

Subject-image distance

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PRE1305Optics glossary.indd 68 01.12.14 13:40 Objective lens

Objective lens is the term used to describe the optical component of a system that is first to absorb the light emitted from the subject and channel it into the system.

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PRE1305Optics glossary.indd 69 01.12.14 13:40 Eyepiece

The final optical element of a microscope that is closest to the human eye is called the eye- piece. The eyepiece acts as a magnifying glass and enlarges the intermediate image gener- ated by the microscope just before the eyepiece once more – albeit as a virtual image. Depending on their design, a distinction is made between monoculars, binoculars and – albeit not quite correctly – trinoculars. Since monoculars offer only one eye opening, the user must close or cover the second eye to obtain a good result. Their simple construction does, however, make monoculars easy to extend, e.g., by adding two pivotable reticules for angle measurement. Binoculars have two eye openings. These do not, however produce a stereo image, they only split the light path to the eyepiece once more and transmit it to two eye openings. Graticules are, by the way, only incorporated into one eye opening on binoculars since both eyes always show the same image and, as such, the composed image is superimposed by a reticule to the human eye. Trinoculars are binoculars with a third opening – which is meant for an additional camera, not for a third eye and therefore does not have a third eyepiece.

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PRE1305Optics glossary.indd 70 01.12.14 13:40 Optically active materials

Materials are optically active when they change the path of a ray of light. These optically active materials include transparent and opaque, i.e., light-permeable and partially permeable materials.

Optical density

When light hits a boundary line between two light-permeable materials, such and light and water, it is refracted, i.e., the ray of light changes direction. This is caused by the differing speeds at which the light wave passes through the various materials or media. Some media have a higher optical density and therefore slow the wave down more strongly than others. The optical density of a material is a material constant and is indicated using the refraction index n relative to the density of the vacuum (n=1).

Material Index Material Index Vacuum 1.000000 (Reference) Sodium chloride (common salt) 1 1.544 Alcohol 1.329 Sodium chloride (common salt) 2 1.644 Amorphous selenium 2.92 Polystyrene (styrofoam) 1.55 Acetone 1.36 Quartz 1 1.644 Chromium oxide 2.705 Quartz 2 1.553 Diamond 2.417 Quartz glass 1.46 Ice 1.309 Ruby 1.77 Ethanol (methylated spirits) 1.36 Sapphire 1.77 Fluorite (Fluorspar) 1.434 Carbon disulfide 1.63 Liquid carbon dioxide 1.2 Heavy flint glass 1.65 Glass 1.5 Heaviest flint glass 1.89 Methyl iodide 3.34 Light flint glass 1.575 Calcite 1 1.66 Emerald 1.57 Calcite 2 1.486 Topaz 1.61 Crystal 2 Water (20°C) 1.333 Crown glass 1.52 Zinc crown glass 1.517 Copper oxide 2.705 Sugar solution (30%) 1.38 Lapis lazuli 1.61 Sugar solution (80%) 1.49 Air (close to the ground) 1.00029

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PRE1305Optics glossary.indd 71 01.12.14 13:40 Plane correction

Geometric aberrations are generally automatically corrected during the achromatic or apochromatic correction of chromatic aberrations. One exception of this is the field curvature. Additional correction, so-called plane correction, is needed to ensure that a flat subject is shown as an equally flat image.

Flat subject

Curved image

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PRE1305Optics glossary.indd 72 01.12.14 13:40 Polarization

An electromagnetic wave is a transversal wave, i.e. the wave oscillates across or – better – per- pendicular to the direction of dissipation. Simply imagine that the direction of dissipation is an axis. A wave that is perpendicular to this axis can curve however it wants around the axis but always remains perpendicular to the axis. Exactly the same applies to light waves that run at all possible angles along the direction of dissipation. If these waves are channeled through a grid consisting of very thin parallel lines that are located very close to each other, light waves that oscillate at a slant or even horizontal to the grid pattern cannot penetrate the grid. By contrast, those rays of light that do pass through the grid all subsequently oscillate in the same direction. They oscillate parallel both to each other and to the grid lines. This alignment of light rays is known as polarization, and light waves that all oscillate in the same direction are known as polarized light. The grid needed to polarize light is called a polarization filter. Polarized light is used in pho- tography or microscopy, for example, to filter unwanted scatter light out of the image or to prevent light reflexes and thus enhance contrasts.

Unpolarized Linear light light polarization

Polarization fi lter

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PRE1305Optics glossary.indd 73 01.12.14 13:40 Polarized analysis / Viewing with polarized light

Microscopes that offer polarized light analysis are fitted with two slots for polarization filters. The first filter is slotted into the light path before the light passes through the objective lens onto the work piece. This ensures that the work piece is only illuminated with polarized light by the microscope. The second polarization filter is slotted behind the beam splitter to channel the light into the objective lens in such a way that only the light reflected by the work piece runs through this filter. The first filter described above is called a polarizer since it generates the polarized light for illu- mination purposes. The second filter is called the analyzer since it is used to filter, i.e., analyze, the reflected light to exactly suit the measurement task. The analyzer rotates, thus permitting various evaluations. If the polarization grids of the polarizer and analyzer are parallel, any scatter light touching the work piece from the side is filtered out. This procedure can be used, for example, to render a strongly reflecting layer beneath the actual surface – which would otherwise show only as a bright spot – virtually invisible and thus to make the underlying structures clearly visible. By contrast, rotating the analyzer so that the polarizer and analyzer grids are at right angles to each other results in the light only being able to pass through the analyzer if its direction of polarization from the work piece is also rotated. Liquid crystal displays are one example that meet this condition. Defective displays can then be easily identified through an omission of rotation; the relevant part of the image remains dark.

Tube lens

Analyzer Polarizer Beam splitter

Objective lens

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PRE1305Optics glossary.indd 74 01.12.14 13:40 Prism

A prism is a glass element with at least two flat polished surfaces that are generally not parallel to each other. The best-known prisms are shaped like an equilateral triangle. But prisms also come in totally different shapes, depending on the task to be performed. A distinction is made between prisms used to perform three different tasks: 1. Prisms for changing the direction of the light path 2. Prisms for splitting the light into individual wavelengths or colors 3. Prisms for high-quality extraction of polarized rays.

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PRE1305Optics glossary.indd 75 01.12.14 13:40 Point diameter / Point size

If a parallel and evenly distributed bundle of light rays is channeled from behind into an infi- nitely corrected objective lens, the objective lens bundles the rays. But even aberration- free objective lenses do not bundle the rays into precisely one point; they form a small circular surface. The diameter and, as such, the size of this surface or point of light depends on the wavelength of the light and the numerical aperture of the objective lens. Since the nor- mal light path is reversed, the point size also indicates the resolving power. This is equally expressed by the fact that the point size is dependent on the numerical aperture. The formula for calculating the size of the measuring point is as follows:

Punktgröße ø (µm) = 1,22 · λ / N.A.

This formula does not, however, apply to laser light, since it is not evenly distributed across the surface. The light intensity of a laser progresses across the beam width in line with the principle of Gauss distribution.

Pupil diameter

Objective lens

Point diameter

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PRE1305Optics glossary.indd 76 01.12.14 13:40 Reflection

Reflection or mirroring is probably the oldest optical phenomenon used by man. Mirrors made of polished metal sheets were used as far back as the Bronze Age in order to strengthen the natural light in rooms. Virtually every surface reflects at least some of the light rays that hit it. Rough surfaces, however, produce diffuse reflection, i.e., the light is reflected without pattern in all possible directions. Even light-permeable materials do not absorb the entire light and always reflect a small part. So-called anti-reflection coating can further reduce the share of reflected light but cannot eliminate it altogether. The very well-known law of reflection applies, whereby the angle at which the light is reflected off the surface is always equal to the angle of incidence (angle of incidence equals angle of emergence).

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PRE1305Optics glossary.indd 77 01.12.14 13:41 Ring light

Ring light is a very common form of incident lighting. The light is shone on to the surface from a ring located around the objective lens. Since this ensures that light is shone evenly on to the work piece from all directions, the illumination is excellent, without any interfering shadows. All areas of the work piece surface are equally well illuminated. Ring light illumination is available in the widest choice of designs. In its simplest form it consists of numerous LEDs arranged in a ring that illuminate the work piece. If the ring light is com- prised of several concentric LED rings, these rings can frequently be controlled independently of each other, thus allowing the angle of light projected by the ring to be varied. The choice of LED color can, moreover, highlight the contrast between different colored areas of the work piece. In the case of the halogen version, the light from the halogen lamp is channeled through an optical fiber into a ring-shaped mirror which then reflects it onto the work piece. Since the optical fiber only transports a small quantity of the heat radiating from the lamp, this type of illumination is also known as cold halogen light. The illumination provided by more sophisticated ring lights can be controlled to such an extent that only a particular segment of the ring light shines onto the work piece. This specific direc- tion of projection enables, for example, the very clear emphasis of edges running parallel to the illumination by means of the shadow cast. This complex ring light technology is primarily used in fully-automated vision measuring equipment.

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PRE1305Optics glossary.indd 78 01.12.14 13:41 Depth of field

Depth of field is the term used to describe a range along the optical axis in which the generated image of the surface is shown to be evenly and optimally focused. The depth of field is dependent on the numerical aperture of the objective lens. A large numerical aperture always results in a small depth of field, whereas the depth of field increases as the numerical aperture becomes smaller. A small depth of field is particularly interesting, e.g., on measuring microscopes, since these perform height measurements by focusing on the different heights. Since the image remains evenly focused in the area of the depth of field, it is a decisive element in the uncertainty of measurements. By contrast, a large depth of field is good when analyzing slightly graduated surfaces, since the entire visible area is imaged at even focus and there is no need to constantly re-focus – depending on which part of the image you wish to view. The term depth of field has become common in microscopy whereas the term depth of focus, which basically describes the same phenomenon, is more common in photography.

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PRE1305Optics glossary.indd 79 01.12.14 13:41 Field number

The field number is an indicator relating to an eyepiece. It can be used to calculate the size of the visible area by multiplying the field number by the magnification of the objective lens. At a magnification of 1x, for example, an eyepiece with a field number of 24 will show a round section measuring 24 mm in diameter of the work piece, whereas at a magnification of 10x, the diameter of this section will only measure 2.4 mm. The field number is usually indicated in the designation of the eyepiece straight after the magnification of the same, e.g., eyepiece 10x/24. Since a higher field number does not change the magnification but rather the visible area, a high field number is much easier on the eye since you don’t have to always concentrate on a relatively small point of light but rather on an image that covers virtually the entire visible area of the eye.

Field number 26 Field number 30

Field number 24

Visible area

The visible area describes the part of work piece that is visible through a microscope. When using an eyepiece, the visible area is usually round, which is why its size is indicated as a diameter. By contrast, if the work piece is imaged by the microscope onto the chip in a digital camera, the visible area is rectangular in line with the shape of the chip. The size of the visible area is, of course, primarily dependent on the magnification of the objective lens. The larger the magnification, the smaller the visible area. The visible area is, however, also limited by the size of the eyepiece, respectively, the camera chip. The intermediate image generated by the objective lens is generally larger than the camera chip or eyepiece, which then accordingly only show a section of the intermediate image.

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PRE1305Optics glossary.indd 80 01.12.14 13:41 Seidel aberrations

Lenses generally have spherical surfaces since these can be quickly, easily and therefore cheaply ground. This is why they are also called spherical lenses. This spherical shape is, however, really not ideal for light refraction. Contrary to theoretical claims, parallel rays of light are not bundled exactly into one focal point if the surfaces of the lens are spherical. The really ideal shape would be parabolic, which is, however, much more difficult to produce. On the other side, spherical and parabolic shapes resemble each closely in terms of symmetry line. So if the size of the spherical lens is limited, the deviation from the ideal shape remains relatively small, but nevertheless still causes imaging errors, so-called geometric aberrations. Physicist Philip Ludwig von Seidel (1821-1896) was the first to systematically examine these geometric aberrations and designated 5 different types of aberration which have since logically become known as the 5 Seidel aberrations: spherical aberration, coma or comatic aber- ration, astigmatism, field curvature and distortion.

True to side image

An image that shows the work piece in the actual same position, without inverting or mirroring it, is called a true to side image. True to side images are useful on optical measuring equipment since the measuring table and work piece generally have to be moved and the direction of movement is only the same for work piece and image when the image is true to side. True to side imaging is absolutely typical of microscopes, whereas it is more the exception in the case of profile projectors. Because of the optical design, true to side imaging on a projector requires sophisticated additional prism optics that are, of course, correspondingly expensive.

Inverted image

An inverted image shows an image of the actual work piece that is rotated by 180°, but not mirrored. Because of their simple optical design, profile projectors generally project an inverted image, whereas this is more unusual in the case of microscopes.

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PRE1305Optics glossary.indd 81 01.12.14 13:41 Spectral properties

Whenever an optical system reacts differently to different wavelengths, the behavior of this system is described using a function curve relating to the wavelengths. In this case, we talk about the spectral properties of an optical system. These properties include, for example, the light permeability of lenses or objective lenses, or the reflection capabilities of a mirror.

266 550 100 80 60 40 20 Transmission (%) 0 200 300 400 500 600 700 800 Visible light Wavelength (nm)

The human eye perceives electromagnetic radiation at wavelengths of between about 380 nm and 780 nm as light in different colors. The visible color spectrum ranges from dark purple to pale red. This range of wavelengths is bordered below by the ultraviolet radiation range and above by the infrared radiation range. Since the terms ultraviolet or infrared light are commonly used, although neither of them is actually visible to the human eye, the wavelengths in the range between 380 nm and 780 nm as mentioned above are called visible light for purposes of bet- ter understanding.

Ultraviolet Infrared

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm

Cosmic Gamma X-radiation Ultraviolet Infrared Terahertz Radar Radio TV Medium wave radiation radiation radiation radiation radiation Long wave Short wave

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PRE1305Optics glossary.indd 82 01.12.14 13:41 Spherical aberration

Spherical aberration is a geometric aberration and one of the 5 Seidel aberrations. It describes the fact that rays of light emitted from a point on the optical axis onto the lens are refracted much more strongly because of the spherical shape of the peripheral surfaces of the lens than is the case in the central area close to the optical axis. This phenomenon prevents the rays of light from rejoining at exactly one point on the optical axis and therefore from creating a clear image of the original point.

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PRE1305Optics glossary.indd 83 01.12.14 13:41 Stereo microscope

What makes stereo microscopes so special is their ability to generate a three-dimensional view of a work piece. Man is capable of seeing his surroundings in three dimensions since the eyes are slightly offset and therefore always capture objects completely separately at slightly different angles to each other. Stereo microscopes support this human perception by possessing two completely separate light paths for each eye which are slightly offset at an angle of 10°-14° from each other. There are two basic designs of stereo microscope – Greenough and parallel or Galileian microscopes. By the way, microscopes with binocular eyepieces are not stereo microscopes since they simply channel the same light path to both eye openings, so the images are not offset against each other and therefore do not create a three-dimensional view.

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PRE1305Optics glossary.indd 84 01.12.14 13:41 Telecentric lens

A telecentric lens is a special feature of an optical system that enables only those light rays that run largely parallel to the optical axis to generate the image. If light rays from subjects located at different distances from the lens touch this lens, each sub- ject will be magnified to a different degree. Since the human eye behaves in exactly the same way, this phenomenon is probably best known and familiar as perspective. If only axis-parallel rays hit the optics, however, it is impossible to identify the distance of origin of these rays. The perspective is therefore not distorted. Since not only magnification but also distance changes the position of the image, a system with a fixed image plane will only show subjects in sharp focus at a specific distance - the other objects - be they closer or farther away - will be shown out of focus. A telecentric system eliminates this loss of focus as well. As such, the design offers a huge advantage when viewing subjects with major differences in height and measuring the subjects in the image – the measurement cannot be corrupted because the perspective distortion is lacking. In order to use only the axis-parallel rays of light to generate the image, a telecentric system has a perforated diaphragm mounted behind the lens in exactly the position of the focal point on the image side. Light shining parallel from the lens to the optical axis is bundled by a lens in the focal point. And only this light can therefore pass through the perforated diaphragm; all other rays are masked.

Standard objective Telecentric objective lens lens

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PRE1305Optics glossary.indd 85 01.12.14 13:41 Telecentric illumination

To a certain extent, telecentric illumination represents a reversal of the principle of telecentric analysis. The use of a perforated diaphragm and lens ensures that the entire visible area of the work piece really is illuminated at even brightness.

Total reflection

On the one hand, a ray of light is refracted at the point where one transparent medium meets another, on the other hand, some of the light is also always reflected. When the light moves from a material with a higher optical density to one with a lower optical density – e.g., when a light ray leaves a glass lens or the water of a lake – the transition can act as a mirror if the angle of incidence is flat, i.e., the light is no longer refracted, it can no longer penetrate the second medium and is, instead, totally reflected. One example for the use of this total reflec- tion is to transport light via optical fibers over a longer distance without massive losses. The light runs at a very flat angle through a glass fiber and is repeatedly totally reflected back from the edge of the fiber into the fiber and transported by this means on down the fiber. The angle of incidence at the end of the fiber is then relatively steep, enabling a very large share of the ray of light to exit the glass fiber. Total reflection can also be proved mathematically, given the applicability of the law of refraction:

sin(a) · n1 = sin(b) · n2

α = angle of incidence, β = angle of emergence, n1 , n2 = refraction indices

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PRE1305Optics glossary.indd 86 01.12.14 13:41 When light moves from water (n1=1.33) to air (n2=1), for example, at an angle of incidence of 48.75°, the angle of emergence is

sin(48,75°) · 1.33 = sin(β) · 1 => sin(β) = 1 => β = 90°

In other words, the light runs exactly parallel to the surface of the water. If the angle of incidence is increased even further, e.g., to 60°, then

sin(β) = sin(60°) · 1.33 = 0.866 · 1.33 = 1.152 .

Since this is impossible, the transition cannot take place; total reflection occurs.

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PRE1305Optics glossary.indd 87 01.12.14 13:41 Transmission factor / Aperture factor

Optical systems are generally very complex. An objective lens, for example, consists of numer- ous lenses with vastly different shapes and made from the widest range of materials in order to create the right combination or produce a good imaging result. Transmission losses occur, however, repeatedly. The quantity of light entering an objective lens will never be the same as the quantity of light emerging from the objective lens.Mounting quality and internal reflection are two of the reasons behind this loss. The ratio of emerging light to entering light is therefore a good indicator for the quality of an objective lens and is frequently indicated in percentage terms as the transmission factor. The transmission factor does, however, have spectral properties, i.e., an objective lens does not transmit all wavelengths of light equally well. An objective lens therefore always has a transmission curve; a function that represents the transmission factor relative to the wave- length.

Spectral transmission rate of a 100x objective lens 100 S

p M Plan Apo NIR 100x ectral transmission factor (%) 80 M Plan Apo SL 100x M Plan Apo NUV 100x 60

40

20

0 300 600 900 1200 1500 1800 Wavelength (nm)

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PRE1305Optics glossary.indd 88 01.12.14 13:41 Tube lens

In the case of an infinitely corrected optical system, the objective lens itself does not gen- erate an image but rather a parallel bundle of rays for each image point. It takes a second lens to rejoin the parallel light paths back into a point and subsequently generate an intermediate image. This second lens is called a tube lens. The distance between objective lens and tube lens is variable, i.e., the magnification of the entire system is independent of this distance.

↓ mechanical components optical elements ↓

constant variable constant Eyepiece Eyepiece

Defl ection Viewing tube prisms

Objective Tube lens lens Tube lens constant variable constant

Microscope Telescope body system Objective Tube lens lens

Objective Objective lens lens

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PRE1305Optics glossary.indd 89 01.12.14 13:41 Ultraviolet radiation

Electromagnetic wavelengths that are slightly shorter than those of visible light are called ultraviolet radiation. The ultraviolet radiation range begins at a wavelength of about 400 nm and less. The lower boundary limit of the ultraviolet radiation range is, by contrast, less clearly defined. Ultraviolet radiation overlaps with X-radiation. The ultraviolet radiation range is sub- divided into different classes. The range from 290 nm to 400 nm is called the near-ultraviolet range since it is close to the range of visible wavelengths. By contrast, ultraviolet wavelengths that are smaller than 190 nm to 200 nm are known as far-ultraviolet radiation. Special objective lenses that are suitable for both visible light and radiation in the ultraviolet range are called ultraviolet objective lenses. These objective lenses are, however, generally not suited equally well to all wavelengths in the ultraviolet range, which is after all very large. Instead, they have a special additional ultraviolet wavelength and are optimized to transmit the same.

Ultraviolet Infrared

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm

Cosmic Gamma X-radiation Ultraviolet Infrared Terahertz Radar Radio TV Medium wave radiation radiation radiation radiation radiation Long wave Short wave

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PRE1305Optics glossary.indd 90 01.12.14 13:41 Ultraviolet microscope

An ultraviolet microscope is specially designed to view a work piece or specimen using ultra- violet radiation. Since ultraviolet light is not visible to the human eye, ultraviolet microscopes are equipped with a special camera that can capture ultraviolet radiation and translate it into a visible image.In addition, ultraviolet microscopes are equipped with special optics since the lenses used for visible light generally only have a relatively low rate of transmission of ultraviolet radiation. It takes the use of special materials, such as quartz glass, to enable a good transmission rate in the ultraviolet range. And, of course, ultraviolet microscopes are fitted with a corresponding special light source that emits large quantities of ultraviolet light. Since optical resolving power is limited, not least, by the wavelength of the light, ultra-violet microscopes always possess a significantly better resolving power – generally twice as good - than conventional light microscopes, given the very small wavelengths in the ultraviolet range. Ultraviolet microscopes are mainly used in medicine and biology.

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PRE1305Optics glossary.indd 91 01.12.14 13:41 Infinitely corrected optical system

An infinitely corrected optical system generates the image of the work piece using an objec- tive lens and an additional tube lens. The objective lens deflects all of the light emitted by one point on the surface of the work piece into a parallel bundle of rays. All of the light therefore runs in parallel bundles between the objective lens and the tube lens and thus does not generate its own image. The tube lens then recombines each bundle of rays into one image point, thus generating the image. The major advantage of an infinitely corrected optical system is its high level of design flexibil- ity. The length of the light path between the objective lens and tube lens is not fixed and can be enlarged or reduced as needed. Accordingly, beam splitters are very easy to use – e.g., to channel incident light into the light path – without having to take the ensuing mismatch into design considerations.

Magnification uncertainty

Manufacturing tolerances can result in the magnified image generated by an optical system deviating slightly from the nominal magnification. In the case of profile projectors, the maxi- mum permissible deviation is indicated in percentage terms as the magnification uncertainty. It is tested by laying a glass scale on the projector’s measuring table to generate an image of the scale. This image is then compared with a second scale superimposed on the image on the screen. The percentage magnification error can be calculated on the basis of the value readings:

L · I · M ΔM(%) = 100 I · M

Mit ΔM(%) = Magnification error of the image L = Value read by the projected scale I = Value read by the reference scale M = Nominal magnification

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PRE1305Optics glossary.indd 92 01.12.14 13:41 Distortion

Distortion is one of the 5 Seidel aberrations and therefore describes a geometric imaging error. Unlike other geometric aberrations, distortion can only occur in a combination of several optical elements, e.g., a combination of a perforated diaphragm and a lens. If the perforated diaphragm is located upstream of the lens, it ensures that light emitting from points to the side of the optical axis can only touch peripheral areas of the lens, i.e., those areas where refraction is somewhat stronger than in the central area. These peripherals parts of the image are therefore moved further into the center of the image as a result, causing barrel distortion, i.e., the optical system distorts a rectangle and shows a barrel shape. A distortion of the image also occurs when the perforated diaphragm is located downstream of the lens. In this case, however, the rays of light emitting from the peripheral areas of the image are masked to such an extent that the resulting rays cause pin cushion distortion of the image. This is therefore a case of pin cushion distortion.

Halogen lamp with condenser

Slide Diaphragm Lens Image

Halogen lamp with condenser

Slide Dia- Lens Image phragm

Slide

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PRE1305Optics glossary.indd 93 01.12.14 13:41 Virtual image

An image that can only be seen when the eye is exactly on the optical axis and looking into the optical system is called a virtual image. The opposite is a real image, which is visible in its position at any time and from any location. A real image is created, e.g., when an image is projected by a slide projector or video beamer. The image is always clearly visible, irrespective of where the viewer is standing.By contrast, an image enlarged through a magnifying glass is only visible when you look straight through the magnifying glass onto the subject. As such, the image enlarged by a magnifying glass is a virtual image.

Mirror

Subject Virtual image

White balance

White balance is necessary before a digital camera can optimally reproduce colors. To achieve this, the camera is “told” which brightness and which color temperature correspond to pure white at the given illumination. This balancing procedure then enables other colors to be identified accordingly and optimally shown on the screen.

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PRE1305Optics glossary.indd 94 01.12.14 13:41 Wollaston prism

A Wollaston prism is a special prism needed for differential interference contrast. This prism is also commonly known as a Nomarski prism or DIC prism (DIC = Differential Interference Contrast).

Centering

When microscopes are fitted with a device for changing objective lenses, e.g., an objective lens revolver, it is important to ensure that the visible area does not move when the objec- tive lens is changed. The center position of the image must remain identical at all levels of magnification. This is achieved by centering the objective lenses. The objective lens with the largest magnification becomes the reference objective lens, and all other objective lenses are bi-directionally aligned to it using corresponding setting screws.

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PRE1305Optics glossary.indd 95 01.12.14 13:41 © MITUTOYO/D 0811 PRE1305

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